This invention relates to medical diagnostic ultrasound catheters. In particular, different materials for constructing medical diagnostic ultrasound catheters provide improvements in dielectric isolation.
Catheters for use with ultrasound systems include transducers for converting electrical energy to acoustic energy and vice versa. Images of a region of a patient adjacent to the transducer are generated. In order to provide accurate imaging, a window or lens is integrated with the catheter adjacent to the transducer. The remainder of the exterior of the catheter generally comprises polyurethane or other biologically acceptable material. In order to provide torque transmission and flexibility, an internal metal braid is provided.
For safe inter-cardiac use of a catheter, the catheter must dielectrically withstand 3,000 volts for 60 seconds between the external surface of the catheter and any internal conductors or electrically active components when internal voltages are greater than 50 volts. Additionally, any leakage current must be less than 50 microamps when 264 volts are applied to the internal conductors. Alternating current capacitive coupling causes a voltage to be applied to the metal braid when voltage is applied to the internal conductors. The capacitive coupling decreases the dielectric withstand strength of the catheter and requires a thicker coating material, such as the polyurethane. However, smaller catheters are generally desired.
Components are often thermally fused with the shaft of the catheter. Since the fusing is thermal, portions of the metal braid, such as exposed end wires, may migrate towards the outside surface of the catheter. A fused catheter more likely will not pass the safety requirements due to migration of the metal braid, resulting in wasted production. The metal braid may migrate to the surface, allowing potential corrosion.
These safety requirements apply to the entire catheter, including the tip area with the transducer. The adjacent lens or window material must satisfy the safety requirements. In order to provide the proper dielectric withstand, the thickness of the lens or window material is increased. However, the required thickness may be considerable. The greater the thickness, the larger the catheter or the less room for transducer components, such as desired additional transducer elements and associated conductors.
To avoid increasing the thickness of the shaft or lens of the catheter, each conductor may be isolated with a transformer. Such isolation requires a different dielectric withstand standard for safety, such as a 300 volt standard. For ultrasound imaging with high resolution, multiple conductors are used, such as 64 to 128 conductors. Isolating each conductor with a transformer is cost prohibitive.